cs: add «684. Redundant Connection»

URL:	https://leetcode.com/problems/redundant-connection/
Signed-off-by: Matej Focko <me@mfocko.xyz>

cs/redundant-connection.cs

@@ -0,0 +1,71 @@
+public class Solution {
+    private List<int>[] MakeGraph(int n, int[][] edges) {
+        var g = new List<int>[n];
+        for (var i = 0; i < n; ++i) {
+            g[i] = [];
+        }
+
+        foreach (var edge in edges) {
+            var (u, v) = (edge[0] - 1, edge[1] - 1);
+            g[u].Add(v);
+            g[v].Add(u);
+        }
+
+        return g;
+    }
+
+    private class DFS {
+        private readonly List<int>[] g;
+
+        private readonly bool[] visited;
+        public bool Visited(int index) => visited[index];
+
+        private readonly int[] parent;
+        public int Parent(int index) => parent[index];
+
+        public int CycleStart { get; private set; } = -1;
+
+        public DFS(List<int>[] g) {
+            this.g = g;
+            visited = new bool[g.Length];
+
+            parent = new int[g.Length];
+            Array.Fill(parent, -1);
+        }
+
+        public void Run(int u) {
+            visited[u] = true;
+
+            foreach (var v in g[u]) {
+                if (!visited[v]) {
+                    parent[v] = u;
+                    Run(v);
+                } else if (v != parent[u] && CycleStart == -1) {
+                    parent[v] = u;
+                    CycleStart = v;
+                }
+            }
+        }
+    }
+
+    public int[] FindRedundantConnection(int[][] edges) {
+        var n = edges.Length;
+        var g = MakeGraph(n, edges);
+
+        var traversal = new DFS(g);
+        traversal.Run(0);
+
+        var cycleNodes = new Dictionary<int, int>();
+
+        var node = traversal.CycleStart;
+        do {
+            cycleNodes[node] = 1;
+            node = traversal.Parent(node);
+        } while (node != traversal.CycleStart);
+
+        return edges.Reverse().First((edge) => {
+            var (u, v) = (edge[0] - 1, edge[1] - 1);
+            return cycleNodes.ContainsKey(u) && cycleNodes.ContainsKey(v);
+        });
+    }
+}